1. Field of the Invention
This invention relates to an actuator for driving a circuit breaker used in an electric power transmission and distribution system, and in particular to a magnetic actuator provided with permanent magnets and electromagnetic coils.
2. Description of the Background Art
FIG. 19 is a diagram generally showing the construction of a conventional electric circuit breaker system 500 of which example is shown in European Patent Publication No. EP0721650 B1.
Referring to the Figure, the circuit breaker system 500 includes a magnetic actuator 100, a circuit breaker 200 which is connected to the magnetic actuator 100 for opening and closing breaker contacts 210, and springs 300 and 301 provided at the top and bottom of the magnetic actuator 100, respectively. These springs 300, 301 assist the working of the circuit breaker 200 when the magnetic actuator 100 causes the circuit breaker 200 to open and close its contacts 210.
FIG. 18 shows principal components of the magnetic actuator 100 of FIG. 19. As depicted in the Figure, the magnetic actuator 100 includes a yoke 250 built up of ferromagnetic laminations, each produced by punching a magnetic steel sheet to form a left-hand yoke section 201, a right-hand yoke section 202, an upper yoke section 203 and a lower yoke section 204. The magnetic actuator 100 further includes permanent magnets 205, an armature 206 which is made movable inside the yoke 250 over a specific stroke, and first and second coils 207, 208. The permanent magnets 205 are attached to solid inner yokes 201b and 202b provided on pole portions 201a and 202a projecting inward from the left-hand yoke section 201 and the right-hand yoke section 202, respectively. The first and second coils 207, 208 used in the magnetic actuator 100 have an equal magnetomotive force (AT). The armature 206 is connected to an actuator rod 209 which passes through the upper and lower yoke section 203, 204 and is joined to the circuit breaker 200. There are provided air gaps g between the armature 206 and the permanent magnets 205. It is to be noted that FIG. 18 shows an example in which the circuit breaker 200 is provided at the top of the magnetic actuator 100 unlike the example shown in FIG. 19.
Let us assume that the armature 206 is currently held at a first position 203a adjacent to the upper yoke section 203 by a magnetic field produced by the permanent magnets 205. When the second coil 208 is excited in such a manner that it produces a magnetic field of the same polarity as the magnetic field produced by the permanent magnets 205, a holding force exerted on the armature 206 by the permanent magnets 205 is canceled out and, as a consequence, the armature 206 moves by as much as the aforementioned specific stroke down to the lower yoke section 204. Then, if the second coil 208 is de-excited, the armature 206 is now held at a second position 204a adjacent to the lower yoke section 204 by the magnetic field produced by the permanent magnets 205. Here, the aforementioned specific stroke of the armature 206 is of an amount which is necessary to break the contacts 210 of the circuit breaker 200, for example.
In the example depicted in FIG. 18, the armature 206 is held at the second position 204a adjacent to the lower yoke section 204, forming an air gap G between the armature 206 and the upper yoke section 203. The spring 301 shown in FIG. 19 assists in opening the contacts 210 of the circuit breaker 200 via the actuator rod 209 when the armature 206 begins to move as a result of excitation of the second coil 208. On the other hand, the spring 300 assists in closing the contacts 210 of the circuit breaker 200 when closing the contacts 210 from an open position shown in FIG. 19.
When the first coil 207 is excited, the armature 206 moves toward the upper yoke section 203 causing the contacts 210 to close and becomes held at the first position 203a adjacent to the upper yoke section 203.
The principle of operation of the armature 206 is now discussed with reference to FIGS. 17A–17C. These Figures also show an example in which the circuit breaker 200 is provided at the top of the magnetic actuator 100 unlike the example shown in FIG. 19.
(1) The contacts 210 of the circuit breaker 200 are in a closed position in FIG. 17A, in which the armature 206 is held at the first position 203a adjacent to the upper yoke section 203 and neither the first coil 207 nor the second coil 208 is excited. The letters “N” in the Figure indicate north poles formed by the permanent magnets 205 on surfaces of the armature 206 and the letters “S” indicate south poles formed by the permanent magnets 205 on surfaces of the pole portions 201a, 202a shown in FIG. 18. Under these conditions, the permanent magnets 205 generate fluxes ΦPM1 and ΦPM2 passing through magnetic circuits L1 and L2, respectively. Since the magnetic circuit L1 has a lower reluctance than the magnetic circuit L2, the flux ΦPM1 is much greater than the flux ΦPM2 (ΦPM1>>ΦPM2), so that a magnetic attractive force occurs between the armature 206 and the upper yoke section 203. This magnetic attractive force is expressed by F=Φ2/S/μ0=Bg2S/μ0, where Bg is the flux density within the air gap G and S is the facing area of the upper yoke section 203 and the armature 206.
(2) When the second coil 208 is excited in this condition, fluxes Φcoil2-1 and Φcoil2-2 are generated as shown in FIG. 17B. These fluxes Φcoil2-1, Φcoil2-2 are combined with the fluxes ΦPM1, ΦPM2 generated by the permanent magnets 205. If a relationship expressed by ΦPM2+Φcoil2-1>ΦPM1−Φcoil2-2 is satisfied, there occurs a force pulling the armature 206 toward the lower yoke section 204.
(3) When the armature 206 comes apart from the upper yoke section 203, the sum of the fluxes ΦPM2+Φcoil2-1 becomes much greater than the sum of the fluxes ΦPM1-Φcoil2-2 (ΦPM2+Φcoil2-1>>ΦPM1−Φcoil2-2), whereby the armature 206 is caused to move by as much as the aforementioned specific stroke and reach the second position 204a adjacent to the lower yoke section 204 as shown in FIG. 17C.
(4) If the second coil 208 is de-excited at this point, the flux ΦPM1 becomes much less than the-flux ΦPM2 (ΦPM1<<ΦPM2), whereby the armature 206 is held at the second position 204a adjacent to the lower yoke section 204 as shown in FIG. 17C.
When the armature 206 moves by as much as the aforementioned specific stroke within the yoke 250 as discussed above, a current flowing in an electric power transmission and distribution system is interrupted by opening the contacts 210 of the circuit breaker 200 which is linked to the actuator rod 209 directly connected to the armature 206.
To bring the contacts 210 from the open position shown in FIG. 17C back to the closed position shown in FIG. 17A, the first coil 207 is excited so that the armature 206 moves up to the first position 203a adjacent to the upper yoke section 203 according to the same principle of operation as described above. The first coil 207 is de-excited at this point and the armature 206 is held at the first position 203a by the flux ΦPM1 generated by the permanent magnets 205, whereby the contacts 210 of the circuit breaker 200 are closed and a current flows normally.
In the magnetic actuator 100 used in the conventional circuit breaker system 500 described above, the permanent magnets 205 for holding the armature 206 at the first or second position 203a, 204a are attached to the pole portions 201a and 202a via the solid inner yokes 201b and 202b, respectively. In this construction, the permanent magnets 205 exist in the magnetic circuits L1 and L2 formed by the first and second coils 207, 208 for actuating the armature 206 and, therefore, eddy currents occur in the permanent magnets 205 and the inner yokes 201b, 202b when an exciting power supply (not shown) is turned on and off.
These eddy currents produce such a problem that they cause not only deterioration of response characteristics of the magnetic actuator 100 but also an increase in the size and cost of the aforementioned exciting power supply.